We consider the following nonlinear Schrödinger equation with derivative: (1) iut = −uxx − i|u| 2 ux − b|u| 4 u, (t, x) ∈ R × R, b ∈ R. If b = 0, this equation is a gauge equivalent form of the well-known derivative nonlinear Schrödinger (DNLS) equation. The equation (1) for b ≥ 0 has degenerate solitons whose momentum and energy are zero, and if b = 0, they are algebraic solitons. Inspired from the works [29, 8] on instability theory of the L 2-critical generalized KdV equation, we study the instability of degenerate solitons of (1) in a qualitative way, and when b > 0, we obtain a large set of initial data yielding the instability. The arguments except one step in our proof work for the case b = 0 in exactly the same way, and in particular the unstable directions of algebraic solitons are detected. This is a step towards understanding the dynamics around algebraic solitons of the DNLS equation. Contents